Material Selection in Mechanical Design

Indiana FIRST / Purdue FIRST ForumsOctober 24, 2015

Matthew PrallSchool of Mechanical Engineering, Purdue University

Overview

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• Product Analysis• Key Concepts• Stress and Strain• Bending and Torsion• Material Selection

Product Analysis

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For a system:• What does it do?• How does it do it?• Where does it do it?• Who uses it?• What should it cost?

For each part of a system:• What is the function?• What does the geometry

look like?• How many are going to

be made?• How is it going to be

manufactured?

Product Analysis

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What is important?• Functionality• Material properties– Mechanical, physical, electrical, ect…

• Geometry• Manufacturability• Cost

Product Analysis

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3 main factors:Loads Material Geometry

Know 2, solve for the 3rd!(usually an iterative approach)

• How strong is a material?– Toughness– Strength– Stiffness– Resilience

• All are different, important, and RELATED terms!

Most important for robot design:Strength Stiffness

Key Concepts

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Key Concepts

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Strength versus toughness• Strength - the resistance of a material to failure due to an

applied stress• Toughness - a material’s ability to absorb energy and plastically

deform without fracturing

Stiffness versus resilience• Stiffness - the resistance of a material to deflection or

deformation due to an applied force• Resilience - a material’s ability to absorb energy when it is

deformed elastically and release that energy upon unloading

Key Concepts

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Important Material Properties:• Strength– Yield (Tensile/Compressive)– Ultimate– Fatigue

• Flexural Modulus• Young’s Modulus• Poisson's Ratio

Material properties are independent of geometry!

Key Concepts

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Material Terminology:• Alloy

– A metallic material consisting of two or more elements that cannot be readily separated

– e.g. Steel (iron and carbon)

• Brittle versus ductile– Brittle materials – little plastic deformation and low energy

absorption before fracture– Ductile materials – extensive plastic deformation and energy

absorption before fracture

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Stress and Strain• Stress: force, or load, per unit area (Pa, psi)

• Can come from: compression (pushing), tension (pulling), shear, bending, torsion (twisting), etc.

Image: Wikimedia Commons

Stress and Strain

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• Strain: how much a material deforms– Variation: normal strain (tension/compression)

• But what are all these letters?– Force (P), length (L), area (A)– Stress (σ), strain (є), deformation (δ)– Young’s modulus (E) [material dependent]

Hooke’s Law

Stress versus Strain Diagram

Stress and Strain

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Bending

• The letters never stop– Moment / torque (M), height in beam (y)– Curvature (C), radius of bend (R)– Young’s Modulus (E) [material]– Moment of Inertia (I) [geometry]

Bending and Torsion

13Image: Wikimedia Commons

*Sign convention: tension is positive, compression is negative

Torsion

• A veritable alphabet soup– Radius (r), angle (θ), length (L), torque / moment (T)– Shearing strain (γ), shearing stress (τ)– Modulus of rigidity (G) [material]– Polar moment of inertia (J) [geometry]

Bending and Torsion

14Image: Wikimedia Commons

Hooke’s Law

Moments of inertia (I and J)• Essentially, resistance to rotation / bending about a

given axis• A cautionary note: There are two types of moment of

inertia: area and mass [we’re concerned with area]• For a Rectangle:

Bending and Torsion

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Tension: Structures stretch under tension• Dependent on material and area• It doesn’t matter what shape• Commonly use cables

Compression: Squeeze under compression• Dependent on material, length, and geometry• Shape matters! Want large value for I• Short sections for compression loads (buckling)

Mechanical Loads

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Bending: Torques / moments can also bend• Dependent on geometry, material• Shape still matters, want large I• Max stress near top (tension) and bottom (compression)

Torsion: Torques / moments can twist• Dependent on length, geometry, material• Again, shape matters! Want large J• Shorter members better resist twisting• Stress concentrated near outside (shear)

Mechanical Loads

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Material Selection

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Common FIRST Materials• Aluminium• Steel• Plastics• Composites

Material Selection

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Aluminium• Common alloys

– 2024– 5052– 6061/6063– 7068– 7075

• Can be heat treated or annealed• Strength and price vary

• Common structural components

Alloy Series Principal Alloying Element1xxx 99.000% Minimum Aluminum2xxx Copper3xxx Manganese4xxx Silicon5xxx Magnesium6xxx Magnesium and Silicon7xxx Zinc8xxx Other Elements

Material Selection

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Aluminum comparison

2024-T4 5052-H32 6063-T6 6105-T5 7075-T6Density (g/cc) 2.78 2.68 2.70 2.69 2.81

Modulus of Elasticity (GPa) 73.1 70.3 68.9 69.0 71.7

Tensile Yield Strength (MPa) 324 193 214 260 503

Fatigue Strength (MPa) 138 117 68.9 95 159

Poisson’s Ratio 0.33 0.33 0.33 0.33 0.33

Material Selection

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Steel• Categories

– Carbon Steel (low, medium, high)– Alloy Steel– Stainless Steel– Tool Steel

• Commonly used for gears, fasteners

Material Selection

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Aluminium versus steel• Cost– Steel generally cheaper per pound (prices vary)

• Strength– Steel is much stronger, but cannot be formed or

machined as easily• Weight– Aluminium is much lighter

• Strength to weight ratio– Aluminum has a higher ratio

Material Selection

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Plastics• Common plastics

– ABS– Polycarbonate– Acrylic

• Much lighter than metals• Lower strength• Non-conductive

Material Selection

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Composites• A combination of two (or more) different materials that

produces a material of superior performance than the components– Fiber and matrix components

• High strength to weight ratio• Tailor strength in specific directions

Material Selection

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Where to find information:• Textbooks• Databooks• Manufacturer’s literature• Internet Sites

Material Selection

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Summary:1. Determine functionality and design requirements2. Calculate loads and geometry3. Choose suitable materials4. Iterate 2 and 3 if necessary

Don’t forget the big picture:

cost versus performance

Concluding Thoughts

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• Balance of cost and performance

• Loads, material, geometry

• Mechanical properties are independent of geometry

• Each material has advantages and disadvantages

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Thank you!

Questions?

References

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[1] Stuart, Jeff. “Designing for Strength and Durability”. School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN. October 2015.[2] NDT Resource Center. “Toughness”. The Collaboration for NDT Education, Iowa State University. October 2015. https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Toughness.htm[3] Campbell, Flake C. Elements of Metallurgy and Engineering Alloys. ASM International. p. 206. ISBN 9780871708670.[4] Engineering and Materials Education Research Group. “Materials Selection for Engineering Design”. University of Liverpool. www.materials.ac.uk/resources/fe/materialsselection.ppt